1. Field of the Invention
The present invention relates to a frequency fine-adjusting apparatus for a piezo-electric oscillator to be used in reference clocks for various apparatus.
2. Description of the Prior Art
A piezo-electric oscillator in which a single crystal of a crystal or the like is used has an extremely large Q factor which is sufficient to ensure a highly stable performance. Therefore, a high-level technique is required for the fine adjustments of the oscillation frequency. Although various oscillation modes exist in piezo-electric oscillators, a thickness slip mode is generally used. In this thickness slip mode, the oscillation frequency of the piezo-electric oscillator is inversely proportional to the thickness of an oscillating reed. It is known that the oscillation frequency can be lowered by making use of the mass additional effect with respect to a main electrode. Conventionally, the oscillation frequency has been finely adjusted by the use of the mass additional effect.
FIG. 4 depicts the construction of a crystal oscillator, which is one of the piezo-electric oscillators.
Referring to FIG. 4, an oscillating reed 20 is provided with a main electrode 21, which is supported on a holder 24 of a lead terminal 23 by a conductive bonding agent 22 so that the main electrode 21 and the lead terminal 23 may be electrically connected to each other. The lead terminal 23 is guided out of a case 26 through hermetic sealing glass 25. After the oscillation frequency has been finely adjusted, a secondary electrode 27 is formed on a central portion of the main electrode 21.
FIG. 5 depicts the construction of a conventional frequency fine-adjusting apparatus for a crystal oscillator.
As shown in FIG. 5, an oscillating circuit 32 is initially connected to a crystal oscillator 28, the oscillation frequency of which generally varies before the frequency fine-adjustment is carried out. The crystal oscillator 28 is then accommodated in a vacuum container 29. After the vacuum container 29 has been evacuated, the oscillation frequency is read by a counter 33 while silver particles 31 for finely adjusting the frequency are caused to adhere to the main electrode 21 through a mask 30 so that the secondary electrode 27 may be formed on the main electrode 21. When a comparator 34 detects that the oscillation frequency has been lowered to a preset value due to the mass additional effect, the vacuum evaporation is suspended by a shutter 35. In this way, the oscillation frequency is finely adjusted.
The frequency fine-adjusting apparatus employing such a vacuum evaporation method requires a high vacuum of approximately 10.sup.-5 Torr and the costs of vacuum equipment are excessive. Furthermore, the operation efficiency is relatively low due to the batch processing.
Japanese Patent Laid-open Application No. 59-13412 discloses a method of raising the frequency by removing the main electrode using a laser.
In this method, the main electrode for imparting an electric field to an oscillating reed is partially removed, and the area to be removed becomes larger as the frequency is adjusted more accurately. As a result, the electric field to be imparted to the oscillating reed becomes weaker and the equivalent resistance is increased. Furthermore, the amount of frequency adjustment which exerts no influence upon the equivalent resistance is relatively narrow in the range of adjustment and cannot allow the change of frequency after the evaporation. Accordingly, this kind of method is disadvantageous in that the frequency adjustment cannot be achieved satisfactorily.